There is substantial evidence that neurons in the midbrain periaqueductal grey region (PAG) play an important role in the modulation of central pain pathways and partially mediate the analgesic effects of systemic morphine. Descending connections from PAG to the medullary raphe region (particularly nucleus raphe magnus) and ascending projections from PAG to the thalamus (including the ventrobasal complex) may be important pathways that underlie these effects. To define better the anatomic organization and functional circuitry of PAG, a series of experiments will be performed to study the efferent connections, transmitter histochemistry, and electrophysiology of PAG cells. Specific projects will: I. Determine whether individual PAG neurons project to both the ventrobasal complex and raphe magnus. The distribution of PAG neurons projecting to the ventrobasal complex and raphe magnus will be studied with double-label retrograde transport methods using horseradish peroxidase and fluorochromes. Double-label techniques will determine whether the same PAG neurons that have descending connections to raphe magnus also project directly to the thalamus. II. Identify the neuropeptide content of OAG neurons that project to the ventrobasal complex or raphe magnus. Numerous studies provide evidence that specific neuropeptides may be important in PAG-mediated somatosensory effects. Combined retrograde transport-immunocytochemical methods will be used to determine which neuropeptides are associated with PAG neurons that project to the ventrobasal complex or raphe magnus. III. Characterize the afferent inputs to PAG neurons. Antidomically identified PAG neurons projecting to thalamus or raphe magnus will be studied using intracellular electrophysiologic methods in anesthetized, paralyzed cats. The orthodromic responses of such cells to shock stimulation of the amygdala, lateral hypothalamus, medullary reticular formation and internal capsule will be determined. The response of PAG neurons to potentially noxious shock stimulation of peripheral nerves and tooth pulp will also be investigated. IV. Study the anatomy of electrophysiologically characterized PAG neurons. Selected, electrophysiologically defined PAG neurons will be intracellularly stained with horseradish peroxidase. The location, somatodendritic arboriztion, and initial axonal projection of these cells will be reconstructed at the light microscope level using camera lucida techniques. Intracellular staining will make it possible to determine whether PAG neurons that differ electrophysiologically are also distinct in their location, appearance, and efferent projections.